The present invention relates to a dual-mode microwave filter for a waveguide intended, for example, for applications in telecommunications satellites. Such filters are capable of presenting filter transfer functions that are very complex and selective.
In the commonest implementation, resonators are used in the form of circular waveguides, together with coupling irises of complex shapes, and each cavity needs to be adjusted manually using a minimum of three adjustment screws.
Dual-mode filters for circular or elliptical waveguides are commonly used in the inlet/outlet networks of communications satellites, and their basic characteristics are well known, e.g. from the article by A. E. Williams xe2x80x9cA four-cavity elliptic waveguide filterxe2x80x9d, published in IEEE Transactions on Microwave Theory and Techniques, Vol. 1.8, (MTT-18), December 1970, pp. 1109-1114, and also in the article by A. E. Atia et al., entitled xe2x80x9cNarrow bandpass waveguide filtersxe2x80x9d, published in IEEE Transactions MTT-20, April 1972, pp. 258-265.
In conventional industrial implementations, a dual-mode filter uses crossed irises to provide inter-resonance couplings and generally presents a minimum of three adjustment screws for each cavity, which screws can be adjusted manually. In addition, because of interactions between coupling irises and adjustment screws, it is necessary to devote considerable experimental effort in order to dimension coupling irises properly.
In order to reduce or even eliminate manual tuning by means of tuning screws, and in order to avoid experimental characterization, it is common practice to a use a software tool to perform a complete simulation of the electromagnetic waves in the final filter structure. As a result, various contributions have recently been made in this field, e.g. by proposing the use of square waveguides, for example as described in the article by Xiao-Pen Liang et al., entitled xe2x80x9cDual-mode coupling by square corner cut in resonator and filtersxe2x80x9d, published in IEEE Transactions MTT-40, No. 12, December 2991, pp. 2994-2302, and in the article by R. Ihmels et al., entitled xe2x80x9cField theory of CAD of L-shaped iris coupled mode launchers and dual-mode filtersxe2x80x9d, published in 1993 in IEEE MTT-S Digest, pp. 765-768.
Other articles have proposed other filter geometries, e.g. the article by R. Orta et al., entitled xe2x80x9cA new configuration of dual-mode rectangular waveguide filtersxe2x80x9d, published in xe2x80x9cProceedings of the 1995 European Microwave Conferencexe2x80x9d, Bologna, Italy, pp. 538-542, or indeed in the article by S. Moretti et al., entitled xe2x80x9cField theory design of a novel circular waveguide dual-mode filterxe2x80x9d, published in xe2x80x9cProceedings of the 1995 European Microwave Conferencexe2x80x9d, Bologna, Italy, pp. 779-783; or indeed in the article by L. Accatino et al., entitled xe2x80x9cA four-pole dual-mode filter realized in circular cavity without screwsxe2x80x9d, published in 1996 in IEEE MTT-S Digest, pp. 627-629.
In addition, tuning screw modeling has been suggested that makes use of a circular waveguide, for example. That modeling is implemented using finite elements as described in the article by Josxc3xa9 Montejo-Garai et al., entitled xe2x80x9cFull-wave design and realization of multicoupled dual-mode circular waveguide filtersxe2x80x9d, published in IEEE Transactions MTT-43, No. 6, June 1995, pp. 1290-1297.
More recently, a very accurate and efficient software tool has been presented for designing and optimizing the entire structure of a filter, including the influence of tuning screws. This is described in articles by Alvarez et al., entitled xe2x80x9cNew simple procedure for the computation of the multimode admittance matrix of arbitrary waveguide junctionxe2x80x9d, published in 1995 in IEEE MTT-S Digest, pp. 1415-1418, and by V. Boria et al., entitled xe2x80x9cAccurate CAD for dual-mode filters in circular waveguide including tuning elementsxe2x80x9d, published in 1997 in IEEE MTT-S Digest, pp. 1575-1578.
Although all of the studies mentioned above have significantly advanced the state of the art in this field, it nevertheless remains that making outlet multiplexers for satellites that are based on dual-mode filters in the form of circular waveguides still requires a great deal of design time and high cost. This is due essentially to two aspects of the design and manufacturing process. The first is that even if the computer-assisted design (CAD) tools that have been developed are indeed practical for designing simple filters, they are not completely suited to designing complex multiplexers having a large number of channels, e.g. 10 to 20. The second aspect is that the required geometry can have shapes that are very complex, and as a result it is very difficult to make such elements physically with the required precision which is generally better than or equal to 2 micrometers (xcexcm) to 5 xcexcm, depending on the electrical specifications.
An object of the present invention is to provide a dual-mode microwave filter which presents the advantages of being simple to design and/or easy to simulate its electromagnetic waves and/or suitable for being manufactured by a method that is simple and low cost.
The invention is based on the idea of using an environment implementing a rectangular waveguide presenting only simple inductive discontinuities.
Given that use is made only of inductive discontinuities in rectangular waveguides, analysis and optimization can be performed in a manner that is much more accurate and efficient than with conventional implementations based on circular waveguides.
Even with complex multichannel multiplexers, design can be performed using known software such as WIND described in the article by M. Guglielmi, entitled xe2x80x9cRigorous network numerical representation of inductive stepxe2x80x9d, published in IEEE Transactions MTT-42, No. 2, February 1994, pp. 317-327, or indeed FEST as described in the article by M. Guglielmi et al., entitled xe2x80x9cA CAD tool for complex waveguide components and subsystemsxe2x80x9d, published in Microwave Engineering Europe, March/April 1994, pp. 45-53.
Another advantage is that the required filter structure is very simple and very suitable for high precision manufacture at low cost, thereby reducing the total cost of development and manufacture in highly significant manner.
The invention thus provides a dual-mode microwave filter, comprising a rectangular resonator of length 1, height b, and width a operating in two distinct modes (m, 0, n) and (p, 0, q) from a single family of modes and presenting the same direction as the E field, and wherein coupling and mode excitation discontinuities are inductive and in the same direction. Said length l and width a are advantageously selected to have a ratio such that said two modes resonate at the same frequency, i.e.:       F    =                                        (                                          m                ⁢                                  xe2x80x83                                ⁢                π                            a                        )                    2                +                              (                                          n                ⁢                                  xe2x80x83                                ⁢                π                            l                        )                    2                    =                                    (                                          p                ⁢                                  xe2x80x83                                ⁢                π                            a                        )                    2                +                              (                                          q                ⁢                                  xe2x80x83                                ⁢                π                            l                        )                    2                          giving:              a      l        =                                        m            2                    -                      p            2                                                q            2                    -                      n            2                              
with m not equal to p and q not equal to n.
The filter can operate in the TEm,0,n and TEp,0,q modes and it is coupled upstream and downstream to first and second rectangular waveguides via openings which are coupled to both of said modes so as to present a transmission zero at the high end of its pass band.
In another aspect, the filter presents a rectangular resonator as defined above, coupled to a monomode resonator, and the ratio between the width a and the length l of said rectangular resonator is selected so that the filter has a transmission zero in the low portion of its pass band.
A dual-mode four-pole filter may comprise first and second rectangular resonators as defined above, which are coupled to each other, the ratios between the lengths I and the widths a of the two cavities being selected so that the resulting dual-mode four-pole filter presents two transmission zeros.
For example, it presents a transmission zero in the low portion of its pass band and a transmission zero in the high portion cf its pass band.
This filter having two transmission zeros may constitute a narrow bandpass filter.
In particular, m=1, n=2, p=3, and q=1.
In yet another aspect of the invention, the rectangular resonator has at least one corner presenting a square or rectangular notch.
In yet another aspect of the invention, the filter comprises third and fourth coupled-together rectangular resonators, and each rectangular resonator includes adjustment screws through a top or bottom wall.
In particular, m=1, n=2, p=2, and q=1